A set of golf clubs includes various types of clubs for use in different respective conditions or circumstances in which the ball must be hit during a golf game. An example set of clubs includes a “driver” for hitting the ball the longest distance on a course, several fairway “woods” for hitting the ball shorter distances than the driver, a set of irons (including one or more “wedges”) for hitting the ball a range of distances that are typically shorter than produced when hitting the ball using a wood, and at least one putter. The term “wood” is based on tradition because such clubs originally were made of wood, but modern clubs of this type are usually made of metal and/or composite materials. The term “iron” also is based on tradition because such clubs originally were made of iron, but modern irons are usually made of steel, other metals, and/or composite materials.
Irons and putters characteristically have a flat (planar) face, wherein the “face” or “striking face” is the surface that normally contacts the ball whenever the ball is being hit with the club. A full set of irons provides lofts ranging from about 18 degrees to about 60 degrees. “Loft” is discussed later below.
A golf club comprises a head (also called a “clubhead”), a shaft affixed to the clubhead, and a grip affixed to the shaft. An exemplary head for an iron 10 is shown in FIG. 3, and includes a face 12, a sole 14, a toe 16, a heel 18, a back 20, a top line 22, and a hosel 24. The sole 14 usually is cambered or otherwise shaped to facilitate a desired interaction between the clubhead and the ground during a swing. The hosel 24 receives the distal terminus of the shaft 26 of the golf club and is the means by which the head 10 is fastened to the shaft 26. The angle of the hosel 24 to the rest of the head 10 is the “lie” of the head 10; during manufacture of irons, the hosel 24 can be manipulated slightly to change the lie to compensate for a golfer's physical characteristics. The face 12 of an iron typically is “offset,” wherein offset is a distance from the front-most part of the hosel 24 to the front-most part, or leading edge, of the head 10. The face 12 typically has a series of score lines (grooves) 28 extending substantially horizontally across the face 12. The particular depth and dimensions of the score lines 28 are regulated by United States Golf Association (USGA) rules because the score lines contribute to the launch conditions of a ball struck off the face 12.
“Loft” is a measurement, in degrees, of the angle at which the face 12 of the clubhead 10 lies relative to a perfectly vertical plane. Through a typical set of irons from the “longest” to the “shortest” iron, the faces of the clubheads have progressively greater loft, which means that the faces are tilted progressively more from vertical. Loft affects the launch angle, backspin, and velocity of a struck ball. Striking a ball with a short iron will typically result in a struck ball having a higher launch angle and greater backspin as compared to a ball struck with a long iron. Consequently, the trajectory of a ball struck with a short iron will typically be higher and shorter than the trajectory of a ball struck with a long iron. To aid the golfer, the irons are numbered to codify the loft; the higher the number, the greater the loft. Generally, the greater the loft, the larger the surface area of the face 12.
Hitting the ball at any location on the face 12 of an iron (or any golf club) does not yield the same result. Every club has a “sweet spot” (a zone located roughly in a central region of the face) that represents the best hitting zone on the face 12 for maximizing the probability of the golfer achieving the best and most predictable shot using the particular club. The sweet spot generally is centered about the center of gravity (CG) of the clubhead, and the smaller the surface area of the face, the smaller the area of the sweet spot. While swinging the club at a ball, the golfer strives to hit the ball inside the sweet spot in a consistent manner so as to provide the greatest probability that the ball will travel in the manner intended by the golfer.
The preferred sizes and masses of the heads of irons have been established by long experience with the playability of iron clubheads. As a result, especially for tournament play, the clubhead of each iron has a characteristic size, shape, and weight. To achieve a desired swing-weight for each club, head-weight standards have been established for each iron. Consequently, someone striving to improve the performance or other characteristic of an iron must work within certain limitations of size and mass. One way in which manufacturers have striven to improve the performance of many golfers using an iron is to increase the size (surface area) of the club's sweet spot without significantly enlarging the face. By using an iron having a larger sweet spot, the golfer can achieve more consistent results shot-to-shot using the club, even if the club does not strike the ball at exactly the same location on the face each time. In other words, a larger sweet spot generally makes the iron more “forgiving” of a golfer's variability in swinging the club and striking the ball with it, thus providing the golfer with a greater assurance of making the intended shot.
Another way of making a club, such as an iron, more forgiving is to increase the moment of inertia (MOI) about the CG of the clubhead, where the CG is the point within the head at which the head is perfectly balanced. MOI is a measure of the head's resistance to a twisting motion caused by striking the ball. For example, if a golfer's swing is off and the ball is struck on the toe of the head, an iron having a higher MOI will exhibit more resistance to twisting caused by the faulty hit, and thus will provide the golfer with a greater probability that the ball will follow the desired flight path.
In view of the size and mass limitations of clubheads, one relatively recent way in which golf-club manufacturers have increased the size of the sweet spot in irons is by removing material from behind the face. These methods tend to reduce the thickness, and thus the mass, of the face, which allows a corresponding redistribution of mass to perimeter regions of the head (called “perimeter weighting”). Perimeter weighting results in a larger percentage of the total mass of the clubhead being situated behind and proximate the perimeter of the face compared to a traditional blade-type iron. This leaves a cavity in regions immediately behind the face, and an iron clubhead having this configuration is designated a “cavity back” iron. Perimeter weighting generally increases the MOI about the CG of the clubhead, resulting in less twisting of the head during off-center hits. An iron with perimeter weighting is typically more forgiving of off-center hits and provides more consistent distance and directional control of a struck ball, resulting in more accurate shots.
Perimeter weighting may also provide latitude for optimal placement of the CG of the clubhead. For most golfers, it is advantageous to place the CG as low in the head as possible to give the struck ball a high launch angle so as to achieve the intended airborne trajectory, and perimeter weighting can facilitate lowering of the CG. Alternatively, the CG can be raised to enable the iron to produce a ball trajectory in which the ball leaves the face at a lower launch angle. Usually, less proficient golfers advantageously use a club having a lower CG, especially when using a long iron.
The “feel” of a golf club embodies characteristics such as sound and vibration transmitted to the golfer as he swings the club and strikes the ball with the club. Feel provides the golfer with various acoustic, tactile, mental, and other feedback from which the golfer can assess performance, game satisfaction, and other criteria closely associated with the golfing experience. The experienced golfer is well acquainted with various stings, shocks, and other types of vibrations transmitted up the shaft from the clubhead that allow the golfer to determine instantaneously whether the ball has been hit within the sweet spot. Manipulating the mass distribution within an iron clubhead can open up possibilities for reducing or dampening stings, shocks, and other undesired vibrations, and thereby enhancing the tactile and acoustic experience associated with making the shot.
The coefficient of restitution (COR) of a clubhead is a measure of the ability of the face of a club to exhibit a springiness or rebound effect that can give the struck ball a bit of an extra push as it leaves the face. The COR effect depends upon the ability of the face to deflect and rebound elastically when the struck ball is still in contact with the face of the club. The maximum COR that can be exhibited by a club is limited by USGA rules.
Particularly with the recent upsurge in popularity of golf, club manufacturers strive ever harder to design clubs that are configured so as to address individual golfers' abilities, strengths, weaknesses, peculiarities of swing, and other factors to provide more (and a greater variety of) golfers with better prospects for an improved and more enjoyable golf game. To this end, a wide variety of club configurations are available, especially of clubs that embody various approaches to manipulating the mass, CG, MOI, COR, feel, and other parameters of the clubheads. In irons, the current latitude for such shifts is dictated largely by the respective densities of available suitable materials from which the heads can be made. Generally, the greater the density of the material, the less the available latitude for shifting of mass distribution and of CG.
Irons traditionally (and mostly still) are made of steel (an iron alloy), such as carbon steel, low-alloy steel, or stainless steel. These steels have a density in the range of 7.7 to 7.9 g/cm3 and have sufficiently high strength for use in irons. Unfortunately, the high density of this material imposes limits on various approaches to performance enhancement. For example, designs for conventional, mass-producible clubs made of steel are limited as to how far down and back the CG can be placed. Manufacturers also have tried various iron designs in which the clubhead is made substantially of steel, but with only the face (strike plate) made of a less dense material such as titanium. Unfortunately, many golfers believe that irons having such a configuration exhibit objectionable “feel” and/or have any of various other shortcomings. Other manufacturers have tried making the entire clubhead of a less dense material such as titanium, aluminum, or composite materials. Unfortunately, these alternative materials usually lack sufficient strength for use in irons, exhibit undesirable COR characteristics, have objectionable feel, require weighting plugs or inserts to achieve a desired mass, are expensive to manufacture, and/or suffer from some other shortcoming.
Many types of clubheads are made by a forging process. Forging worked well for earlier, more conventional, club-head designs. However, forging oftentimes is incapable of producing complex clubhead geometries and configurations, such as cavity-back designs. Additionally, with the recent advent of more highly “engineered” clubheads, it now is desirable that the heads be formed to tighter tolerances than are possible using forging processes to minimize expensive downstream machining steps. As a result, club manufacturers have employed various casting methods, especially investment casting, with good results using the several high-density steel alloys commonly found in clubheads, particularly irons.
Various specific attempts at developing lower-density steel alloys and other materials for use in golf clubs are described in the following references. U.S. Pat. No. 6,685,577 to Scruggs discusses clubheads made of an amorphous metal containing 45-67 at % Zr+Ti (zirconium and titanium), 10-35 at % Be (beryllium), and 10-38 at % Cu+Ni (copper and nickel). U.S. Pat. No. 2,931,098 to Johnson discusses irons made of alloys consisting predominantly of Cu and either Zn (zinc) or Al (aluminum). U.S. Pat. No. 6,520,868 to Chen discusses clubheads made of a steel alloy containing (by weight) maximally 0.03% C (carbon), 0.2-0.6% Si (silicon), maximally 0.15% Mn (manganese), maximally 0.03% P (phosphorus), maximally 0.03% S (sulfur), 10.5-13.5% Cr (chromium), 0.8-1.4% Mo (molybdenum), 0.8-1.4% Ni, 0.02-0.1% Nb (niobium), maximally 0.01% N (nitrogen), maximally 0.03% Cu, and the balance being Fe. U.S. Pat. No. 4,314,863 to McCormick discusses clubheads made of a steel alloy containing (by weight) 13-20% Cr, 2.0-3.6% Ni, 2.0-3.5% Cu (with sum of Ni and Cu being at least 5.0%), 0.2-1.4% Mn, 0.5-1.0% Si, maximally 0.035% P, maximally 0.035% S, less than 0.10% niobium (Nb), less than 0.10% Al, 0.20-0.80% C (with maximally 0.05% N) or 0.10-0.60% C (with 0.05-0.10% N), and the balance being Fe. U.S. Patent Application Publication No. 2003/0082067 A1 to Chao discusses clubheads forged of an iron alloy containing (by weight) 28-31.5% Mn, 7.8-10.0% Al, 0.90-1.10% C, 0.35-2.5% Ti (titanium), and the balance being Fe. U.S. Pat. No. 6,617,050 to Chao discusses clubheads made of an alloy containing (by weight) 25-31% Mn, 6.3-7.8% Al, 0.65-0.85% C, 5.5-9.0% Cr, and the balance being Fe. U.S. Pat. No. 5,167,733 to Hsieh discusses clubheads (specifically drivers) made of an alloy containing (presumably by weight) 0.5-2.0% C, 25-35% Mn, 5-10% Al, 0.5-1.5% Mo, and the balance being Fe. In Hsieh, the alloy is used to fabricate, by casting, the entire head of the driver including the face, which necessitates making the head in two parts that must be welded together. Unfortunately, the performance and feel of such a head are not satisfactory for many players.
In view of the foregoing, there remains a need for further improvements in methods for making clubheads (especially irons) that have the desired latitude for mass distribution, CG shifting, and other configurational manipulations for achieving optimal performance and feel.